Can my son take Vyvanse (lisdexamfetamine) for ADD, even though he was just diagnosed with Rt bundle branch block? If not, can you recommend any natural supplements?

None. natural supplements have no scientific values. If your son has right bundle branch, a cardiology consult is needed. He may still take vyvanse (lisdexamfetamine) after careful review.
You need clearance . from his pediatric cardiologist 1st. RBBB may occur alone or with Atrial Septal Defect/other Congenital Heart Disease, cardiomyopathy or Hypertension. Natural supplements are ineffective in treating ADHD. Some, like St. John's Wort & energy drinks increase BP. All stimulant & non-stimulant meds affect heart rate & BP. See for how to get him a Section 504 at school & behavioral therapy. .
Cardiologist. prior to starting any new medication, whether prescribed by your pediatrician, psychiatrist, or if you buy it over-the-counter, you should discuss it with your child's cardiologist. Just because something is labeled as "natural" and sold in a "health-food type store" does not mean it is safe. Coordinate all meds and supplements with pediatrician and cardiologist.